1. Field of the Invention
The present invention relates to a method for forming a blank spike which is cut with threads after its formation. More particularly, the present invention relates to an efficient, accurate and economical method for forming a blank railroad spike using a series of cold forming dies.
2. Description of Related Art
Railroad spikes are used to fasten a rail to a wooden tie. An efficient method must be provided to manufacture economically the enormous number of spikes necessary for construction or repair of a railraod.
A commonly used railroad spike is known as the "Tirefond" or "North American" spike screw which is illustrated in FIG. 3. In its finished state, the spike has a shank 12 with a cylindrical portion 14 and a tapered portion 16 which decreases in diameter towards the tip of the spike. The shank is cut with a particular thread arrangement which cooperates with the tapered portion to firmly grip the wooden tie and prevent pull-out. The head 10 of the spike is provided with a squared end that receives a tool for screwing the threaded spike into the tie. When fully screwed into the tie, a collar 18 located below the squared end caps the screw hole to prevent ingress of water or other corrosives. Such a railroad spike is illustrated in U.S. Pat. No. 4,278,374.
One method of forming the "Tirefond" or "North American" spike is by a hot forging and rolling process in which a heated cylindrical bar is inserted between rotating drums to form the spike with the squared head collar, tapered shank and threads. For example, see British Patent Specification No. 757,709, filed May 12, 1953. However, this process is cumbersome and expensive, as well as being unreliable in terms of the accuracy and precision of the dimensions of the spike and thread.
As an alternative to hot forging and rolling, a blank or unthreaded spike is first formed with a squared end, collar and cylindrical shank. The blank spike is subsequently cut by a milling operation to form a tapered shank with threads. A milling apparatus for tapering the shank and cutting the thread is disclosed in U.S. Pat. No. 4,278,374. Alternatively, a device employing tangential chasers to cut a thread on a blank railroad spike may be employed as disclosed in the above-identified related application.
Regardless of the device used to cut the thread on a blank spike, the formation of an accurate, unblemished blank spike is critical to the economical manufacturing of the finished spike, since it is useless to cut a thread on a defective or faulty blank spike. The present invention is directed to the economical manufacture of a blank spike.
One such known process for forming a blank spike is illustrated in FIGS. 1A-1D. In that method, a wire is drawn to an initial diameter (preferably about 0.907 inches) and passed through a cut-off station which cuts the wire into sections of approximately 7 inches. In FIG. 1A, the wire section is introduced into a first die in which a punch upsets or "bumps up" the top of the wire to increase its diameter to preferably about 1.111 inches. The first die forms a first stage spike having a shank 20 and a head 22. The head 22 includes a tapered portion 24 which is formed by compression of the wire by the punch. The diameter of the tapered portion increases between the shank and the top of the head 22. Preferably, the initial diameter of the shank is not affected by the punch and therefore remains at about 0.907 inches.
The first stage spike of FIG. 1A is introduced into a second die to form a second stage spike illustrated in FIG. 1B. In the second stage spike, a punch upsets the head 22 and tapered portion 24 to form a rough squared end 30 and a bulge section 32 beneath the squared end 30. In other words, the squared end 30 is formed by "bumping up" the top of the first stage spike. The squared end 30 has rounded edges which must be smoothed into corners so that a tool can be applied to the spike. The bulge section 32 has a diameter equal to that of the tapered portion of the first stage spike (i.e., 1.111 inches). The initial diameter of the shank is a not appreciably modified by the second die, but may be upset to about 0.912 inches.
The second stage spike is introduced into a third die to form the third stage spike illustrated in FIG. 1C. In the third stage, the bulge section is compressed to form a collar 40. Preferably, the diameter of the collar 40 is approximately two inches. In addition, the third die smoothes the squared end 30 to form corners for the tool, while forming a depression 42 in the squared end and an underfill area 44 at the lower tip of the shank. After forming the third stage spike, the spike may be introduced into a fourth die (FIG. 1D) which provides a recess 46 in the base of the shank for the holding the blank spike during the thread forming operation.
This known method has a major disadvantage. In forming the blank spike, the top end of the wire is first built-up to form the rough squared end and bulge section, thereby resulting in a significant amount of metal movement which creates fatigue in the spike. The bulge section then is compressed to form the collar while the squared end is smoothed. The head and collar area are overworked by the build-up and compression so that tensile ruptures, stress cracks and other fractures may form therein. For example, upsetting the wire from its initial diameter (e.g., 0.907 inches) to the diameter of the collar (e.g., 2.0 inches) usually exceeds the ability of the material to deform in about 33% of all spikes. In addition, even the portions of the spike that have not been ruptured still show indications that the material has been severly overworked. The overworking of the material results in an unsightly or blemished spike.
In view of the foregoing, only an average of about 65% of the spikes formed in accordance with the above-described method are acceptable for the thread forming operation. Such a low percentage is extremely uneconomical when manufacturing enormous numbers of spikes.
It is an object of the present invention to obviate the above-described disadvantage by providing an economical spike forming method which has an average acceptance percentage of about 90-95%.
Another object of the present invention is to provide a spike forming method which does not overwork the material in order to form the squared end and collar.
It is a further object of the present invention to provide a spike forming method which provides better head and collar formation by inhibiting the formation of ruptures, cracks and fractures.
In accordance with the presently claimed invention, a method is provided for forming a cylindrical workpiece having a first diameter into a blank spike which is subsequently provided with threads. The method includes the steps of introducing the workpiece into a first die to form a first stage spike having a cylindrical shank and a tool receiving end. The shank of the first stage spike has upper and lower portions of a first diameter which is equal to the diameter of the cylindrical workpiece. The first stage spike is introduced into a second die to form a second stage spike. The second die reduces the lower portion of the shank of the first stage spike to a second diameter which is less than the first diameter of the upper portion of the shank. The second stage spike is introduced into a third die to form the third stage spike. In the third stage, the upper portion of the shank of the second stage spike is compressed to form a collar between the tool receiving end and the lower portion of the shank.
The inventive method for forming a spike provides significant advantages over the known method. In the inventive method, the upper portion of the first stage spike has a diameter sufficient to form the squared head without being "bumped up". Thus, the upper portion of the first stage spike does not need to be upset to increase its diameter thereby resulting in less metal movement and less working of the material. In addition, the first die forms the first stage spike with a tool receiving end which is substantially the same as the tool receiving end of the finished blank spike. Accordingly, the upper portion of the shank and the tool receiving end are not overworked so that the formation of cracks and/or fractures are inhibited.